The invention is related to a distance holder for connection to, and rotation with, a drill string in an earth formation drilling device arranged to supply a jet of abrasive fluid for the purpose of providing a borehole by removing earth formation material through abrasion, comprising a housing with a chamber which is essentially rotational symmetric and which is to face the earth formation material, and a jet nozzle which arranged for discharging a jet of the abrasive fluid in said chamber, said housing comprising at least one slot for allowing the abrasive fluid to leave the chamber.
Such a distance holder is disclosed in WO-A-2005/040546. By means of an earth formation drilling device which is equipped with a distance holder of this type, the borehole bottom is abraded by the abrasive particles comprised in the abrasive fluid which is discharged at high velocity. Due to the orientation of the jet nozzle, a cone is formed on the borehole bottom. The abrasive fluid hits said cone, abrading it further and further. The fluid is discharged from the chamber through the slot, and subsequently the fluid is urged to flow upwardly along the outside of the distance holder into the annulus between the drill string and the borehole wall. By means of a magnet contained in the earth drilling device, the abrasive particles are extracted from the fluid and fed back to the jet nozzle for further abrasive action.
However, the shape of the cone and the way in which the fluid hits said cone, may impair the extraction of steel abrasive particles. The steel abrasive particles show the tendency to roll along the slope of the cone formed on the borehole bottom. The rotational speed of these steel abrasive particles may well exceed 60,000 rpm in this way. The steel abrasive particles continue to rotate at this high rotational speed while travelling upwardly along the earth drilling device and in particular along the part thereof containing the magnet.
The rotation of the particles has a tangential orientation. The contacts of the rolling particle with the borehole wall further induces the rotational effect with tangential orientation. Said rotation of an abrasive particle that contains ferromagnetic and electrically conducting material reduces the penetration of a magnetic field into the particles. This causes a reduction of the magnetic force exerted by the magnetic separator onto the steel abrasive particles. For instance, in the case of steel abrasive particles with a diameter of 1 mm, the loss of magnetic attraction becomes significant. The combination of upward particle velocity and rotational particle speed at the position of the magnetic separator makes the magnetic field generated by the magnetic separator less effective. Consequently, extraction of the steel abrasive particles from the fluid is impaired.